Chip-Scale Atomic Clocks (CSACs) contain vapors of alkali metals, typically either rubidium (Rb) or cesium (Cs), which are located in vapor cells. The walls of the vapor cells are typically made from glass and silicon. A bichromatic (2 wavelength) optical field is sent through the vapor, exciting hyperfine transitions using a phenomenon called coherent population trapping. A rubidium-based CSAC, for example, works by exciting the D1 hyperfine transition using a vertical-cavity surface-emitting laser (VCSEL) that is tuned to the broad absorption at 795 nm and radio frequency (RF) modulated at 3.417 GHz—precisely half the D1 transition frequency.
It is known that some CSACs become inaccurate when the ambient temperature changes. This is due in part to the fact that two of the components in the CSAC, the vapor cell and the VCSEL, are not operated at their most stable temperatures.
One of the more challenging aspects of making CSACs for military applications involves dealing with the temperature variations that real military systems encounter during use. Even though attempts have been made to stabilize the temperature of the most sensitive components, the temperature sensitivity of the CSACs can still be a problem. For example, radiative coupling and gas-phase thermal conduction from the components inside the physics package to the walls of the package can cause the temperature of those components to change.
In order to minimize the temperature sensitivity in CSACs, both the vapor cell and the VCSEL need to be operated at points where dFCSAC/dTcomponent is zero. However, the temperature where dFCSAC/dTRb-cell is zero and the temperature at which the VCSEL outputs the proper wavelength of light is in general different. In attempting to thermally anchor these two components together as is traditionally done, maintaining CSAC stability over temperature can become a very big problem.